The present invention relates to induced-draft, gas-fired furnaces, and is directed more particularly to an improved induced-draft, gas-fired furnace which is adapted to operate at any of three different firing rates.
An important consideration in the design of multi-stage furnaces, i.e., furnaces which have more than one firing rate, is the quantity of air per unit time which they circulate through the space to be heated at each of those different firing rates. This quantity of air, commonly referred to as the circulating airflow of the furnace, is preferably as low as the heating requirements of the space to be heated permits. This is because lower speeds are associated with quieter operation. Circulating airflows which are relatively lower, but which continue for relatively longer periods of time, are also desirable because they provide greater thermal comfort.
Quietness of operation and thermal comfort, however, are only two of many things that must be considered in designing multi-stage furnaces. Other considerations include the annual fuel utilization efficiency or AFUE of the furnace, the furnace operating cost, and flue gas emission requirements. Still another consideration is the requirement that the heat exchanger of the furnace be protected from "cold spot corrosion" either by maintaining it at a temperature too high for water to condense thereon, or by making it from a corrosion resistant material. Because these requirements conflict with one another, tradeoffs must be made.
Examples of furnace designs which make various kinds of tradeoffs are described in U.S. Pat. No. 4,708,636 (Johnson), U.S. Pat. No. 5,248,083 (Adams et al), U.S. Pat. No. 5,307,990 (Adams et al), and U.S. Pat. No. 5,590,642 (Borgeson et al). The Johnson patent describes a furnace which includes a modulating gas valve and furnace controls that are responsive to a flow sensor that is mounted in the stack. This patent is concerned primarily with maintaining the proper fuel-to-air ratio and a predetermined minimum airflow in the stack, however, and does not take into account the importance of multiple stages and their effect on thermal comfort and noise levels.
The Adams et al 083 patent describes a furnace in which a modulating gas valve is used to control the firing rate of the burner in accordance with the temperature of the heat exchanger, and in which the blower speed is changed as necessary to match the delivery of heat to the heat load on the furnace. The furnace described in this patent, however, uses an adaptive control algorithm which tries to maintain constant on/off times by regulating the blower speed based on previous on/off cycle data for prior heating cycles and is not coordinated with the speed of the induced draft blower. As a result, the efficiency, operating cost, and noise level of the furnace are not optimized.
The Adams et al 990 patent also describes a furnace in which a modulating gas valve is used to control the firing rate of the burner in accordance with the temperature of the heat exchanger. In this furnace, however, the blower speed is continuously adjusted as necessary to maintain a constant differential pressure across the heat exchanger, but is still not coordinated with the speed of the induced draft blower. As in the case of the Adams et al 083 patent, the efficiency, operating cost, and noise level of the furnace again are not optimized.
The Borgeson et al patent describes a furnace in which the state of a modulating gas valve and a variable speed induced draft blower are both varied as necessary to maintain the temperature of a heat exchange medium at a constant value while the blower speed is varied to maintain a constant circulating air temperature. In this case temperature sensors are used and these tend to react in a very unpredictable manner from one installation to the next because they are location and airflow sensitive and do not account for all possible variations.
In an earlier filed U.S. patent application Ser. No. 09/407,052, filed Sep. 27, 1999, which is commonly assigned herewith, and which is hereby expressly incorporated by reference herein, there is described a method and apparatus for increasing the low stage circulating airflow of a furnace without creating conditions that give rise to cold spot corrosion. Generally speaking, this method involves the maintaining of a predetermined relationship between the low stage circulating airflow of the furnace and the low stage combustion airflow thereof, i.e., the rate at which the induced draft blower supplies combustion air to the burner. By maintaining a predetermined relationship between the magnitudes of the combustion and circulating airflows, the temperature at the outlet of the heat exchanger is kept substantially constant at a value high enough to prevent water from condensing thereon and causing cold spot corrosion.